It’s really common to get asked to control the humidity in walk-in coolers. I’ve been asked to provide everything from 100 percent RH to 10 percent RH in some specialized applications. While, it is somewhat true that everything is possible, controlling refrigeration systems for humidity can be challenging and energy intensive.
We don’t have the space to do a full-blown psychrometric review here, but let’s take a quick detour to understand what exactly humidity is because a lot of problems come from misunderstanding the terminology we use.
Said simply, psychrometrics is engineering and science concerning air with water in it. As almost everyone knows from watching a weather forecast, air can hold water vapour; we know, for example, that air that is hot and humid feels warmer than hot dry air. The weather people will use the term humidex to describe this effect. We aren’t really concerned about the humidex here (although understanding it actually does matter when it comes to outdoor fresh air economizing using temperature or enthalpy controls… perhaps a topic for another day).
It turns out that the amount of water that air can hold is directly related to its temperature. Hot air can hold significantly more moisture than cold air and you can see this in Figure 1 which shows the amount of water in grams of water per kilogram of air that air can hold at temperatures ranging from -20C to 40C.
In general, if we try to add more water to the air than it can hold, we end up with things like fog, mist, and clouds. The term relative humidity (RH) is simply the ratio between how much water is in the air we are thinking about compared to how much it could hold.
Every point on the curve on Figure 1 is at 100 percent RH even though the air at -20 has 75 times less actual water in it. If you are familiar with psychrometric charts you will likely recognize the shape of that curve.
So, it is important to be really clear on what the person actually wants when they are asking for particular humidity in a walk-in cooler. As you can imagine, looking at Figure 1, in order to have low RH at low temperatures, there has to be an extremely low amount of moisture in the air.
Establishing the dew point
The next term we need to make sure we understand is the dew point. The dew point is the temperature at which the air you are considering would be at 100 percent RH. Said another way, it is the temperature at which dew (water condensation) will form if the air touches it. We are all familiar with this in many contexts outside of refrigeration and we discussed this in the last article about defrost and frost formation.
At 100 percent RH the dry bulb temperature of the air is the dew point. Figure 2 shows where 20C air with a 50 percent RH would show up on the chart from Figure 1. Air at these conditions has about 7.3g/kg of water.
Following a horizontal line straight over to the curve we can see that air at approximately 10C will be at 100 percent RH with that much moisture (Actually the answer is 9.3C, but close enough…). This means that if we took that air and cooled it to 10C it would be at 100 percent RH.
Alas, you may want to point out to me right now that cooling that air to 10C without having a surface colder than 10°C, will require an infinitely large heat exchanger. And you’d be right. So, in order to cool it to 10C we will need a colder surface and then condensation will happen, which will remove the moisture from the air. We can therefore see that the temperature we choose for our heat exchanger will affect the resulting humidity in the space.
The other significant factor that will determine how much moisture is removed by cooling is the actual geometry of the coil. You can imagine that for coils where a lot of air passes without contacting any surface and/or the air passes really quickly, less moisture will be removed.
Engineers doing proper heat load calculations for some commercial, institutional and industrial buildings will often figure out what the latent load (the load caused by removing water) and the sensible load (the load caused by changing temperature) and actually design the coil temperature and geometry to match the design requirements.
With a few very special exceptions, this is not required in commercial refrigeration (and is actually really hard to do because most manufacturers do not provide adequate information about their coils).
The result is this – in commercial refrigeration, the only factor you “choose” is the temperature of your refrigerant. The difference between this temperature and the air is often referred to as ‘TD’.
There are actually two ways of calculating TD and understanding coil ratings but, for now, let’s assume the TD is between the average box temperature and the coil. So, in a 4C walk-in cooler, the refrigerant would be at -2C if there was a 6C TD.
The smaller TD you choose, the higher the RH will be maintained in your box because the coil will remove less water. If you choose a larger TD, then the RH will be lowered.
The question is how can you tell where you will end up? And the answer is that you can’t unless you are going to do a lot of work to understand infiltration and where the moisture load is coming from in your application.
What you can easily do is look to some averages and “normal” results. Typically, if you size a walk-in box with a 5.6C TD, you will end up in the 80 to 85 percent RH range. If you go lower, say around 4C (8F), the RH could be in the 90 percent range. Higher, of course, the opposite happens.
There are limits since you can only remove moisture if the temperature of your coil is colder than the dewpoint of the air you are trying to create. Figure 3 shows how the dew point temperature decreases as a function of RH in the room. So, in order to remove any moisture at 4C and 50 percent RH, the coil would have to be colder than -6C as a theoretical max temperature. Truthfully it would likely have to be colder.
None of what we have discussed so far talks about controlling humidity. Consider the last example of the walk-in at 4C and 50 percent RH. We could theoretically achieve this with a coil of -6C or lower.
However, what’s going to happen if we use a humidistat instead of a thermostat? Most of the time… you will end up with a really cold box. This is because we can’t avoid cooling the air too much in order to remove the water.
The only way to counteract this is to reheat the air we cooled too much. This is precisely what portable dehumidifiers do. They simply use the refrigerant condenser as a reheat. which is also why they also add heat to the room.
The bottom line is that mechanical refrigeration systems can make excellent dehumidifiers, just be careful making promises about conditions that are going to be very hard to achieve. The next time someone tells you they want -15C and 10 percent RH, make sure they know what they are talking about (The coil has to be colder than -40C to achieve a humidity this low).